Design Online - November 2010
Improving nanoscale Raman-AFM imaging with
negative-stiffness vibration isolation
The need for precise vibration isolation with scanning probe
microscopy (SPM) and nearfield scanning optical microscopy
(NSOM) systems is becoming more critical as resolutions continue
to bridge from micro to nano. Whether used in academic labs
or commercial facilities, SPM and NSOM systems are extremely
susceptible to vibrations from the environment.
When measuring a very few angstroms or nanometers of displacement,
an absolutely stable surface must be established for the instrument.
Any vibration coupled into the mechanical structure of the
instrument will cause vertical and/or horizontal noise and
bring about a reduction in the ability to measure high-resolution
features. The vertical axis is the most sensitive parameter
for SPMs, but these instruments can also be quite sensitive
to vibrations in the horizontal axis.
| In a negative-stiffness isolator,
vertical-motion isolation is provided by a stiff spring
that supports a weight load, combined with a negative-stiffness
mechanism. Beam-columns connected in series with the vertical-motion
isolator provide horizontal-motion isolation. Diagram
courtesy of Minus K Technology Inc.
Lab design teams obviously need to plan for these special
equipment requirements, as they make decisions regarding building-level
isolation techniques and localized techniques. Traditionally,
bungee cords and high-performance air tables have been the
vibration isolators most used for SPM and NSOM work. The ubiquitous
passive-system air tables, adequate until a decade ago, are
now being challenged by the more refined imaging requirements.
Bench-top air systems provide limited isolation vertically
and very little isolation horizontally.
Also at a disadvantage are active isolation systems, known
as electronic force cancellation, which use electronics to
sense motion and then implement equal amounts of motion electronically
to compensate and cancel out the motion. Active systems are
somewhat adequate for applications with lasers and optics,
since they can start isolating as low as 0.7 Hz. But because
they run on electricity, they can be negatively influenced
by problems of electronic dysfunction and power modulations,
which can interrupt scanning.
Lately, the introduction of integrated microscopy systems
employing multiple scopes is enabling more complex optical
measurements, but these systems are also much heavier, and
there has been little vibration-isolation technology available
for such heavy instrumentation. Air tables, which have been
liberally used for optics applications, are not ideal for
these nano-scale resolution systems because of their inability
to effectively isolate vibrations below 20 Hz. Nor can active
systems be used with these newer combination systems because
of their inability to handle heavy instrumentation.
Negative-stiffness mechanism (NSM) vibration isolation offers
a viable alternative choice for SPM and NSOM systems. This
includes applications using atomic force microscopy (AFM)
integrated with micro-Raman spectroscopy, where negative-stiffness
vibration isolation is particularly well-adapted. In fact,
it is the application of negative-stiffness isolation that
has enabled AFMs to be truly integrated with micro-Raman.
Negative-stiffness isolators can handle the weight of a combined
system, as well as isolating the equipment from low-frequency
vibrations: a critical set of factors that high-performance
air tables and active systems cannot achieve.
The neuronal sample is derived from slices of rat neocortex.
AFM with micro-Raman integrated
The integration of AFM with micro-Raman enables a sizable
improvement in data correlation between the two techniques,
and expanded Raman measurement and resolution capabilities.
Micro-Raman is a spectroscopic NSOM technique used in condensed
matter physics and chemistry to study vibrational, rotational
and other low-frequency modes in a system. It relies on scattering
of monochromatic light-usually from a laser in the visible,
near-infrared or near-ultraviolet range. The laser light interacts
with phonons or other excitations in the system, resulting
in the energy of the laser photons being shifted up or down.
The shift in energy gives information about the phonon modes
in the system.
Scanning samples in a micro-Raman system, however, entails
problems. As a sample is scanned, even a very flat sample,
it is hard to keep the lens-to-sample distance constant. As
one goes from pixel to pixel under the lens of a Raman, a
mixture of sample and air is sampled in the illuminated voxel
(volumetric picture element). This causes "artifactual"
intensity variations in the Raman that are unrelated to the
chemical composition of the sample. The effect is even more
pronounced with rough samples.
Standard methods of auto-focus are simply not accurate enough
for a host of problems being investigated. Additionally, the
point spread function, which determines the resolution of
the Raman image, is significantly broader where there are
contributions from the out-of-focus light. and this reduces
The atomic force microscope, a very highresolution type of
scanning probe microscope, has demonstrated resolution of
fractions of a nanometer, making it one of the foremost tools
for imaging, measuring and manipulating matter at the nano
scale. The information is gathered by "feeling"
the surface with a mechanical probe. Piezoelectric elements
that facilitate tiny but accurate and precise movements on
electronic command enable the very precise scanning.
The AFM consists of a micro-scale cantilever with a sharp
tip (probe) at its end that is used to scan the specimen surface.
The cantilever is typically silicon or silicon nitride, with
a tip radius of curvature on the order of nanometers. When
the tip is brought into proximity with a sample surface, forces
between the tip and the sample lead to a deflection of the
cantilever. Resultant characteristics- mechanical, electrostatic,
magnetic, chemical and other forces-are then measured by the
AFM, using, typically, a laser spot reflected from the top
surface of the cantilever into an array of photodiodes.
Most systems employing AFM plus Raman execute the two types
of scans independently. The recently developed direct integration
of Raman spectroscopy with AFM technique, however, has opened
the door to significantly improved technique and sample analyses.
Micro-Raman is a micro-technique, but when AFM is added, it
becomes a nano-technique. It allows the AFM structural data
to be recorded on line and improves the resolution of the
Raman information when the nanometric feedback of the system
adjusts, with unprecedented precision, the position of each
pixel of the sample relative to the lens. The small movements
of the AFM stage also provide oversampling: a well-known technique
for resolution improvement.
One integrated AFM-Raman system developed by Nanonics Imaging
Ltd. (in association with major Raman manufacturers such as
Renishaw plc, Horiba JY and others) provides simultaneous
and, very importantly, on-line data from both modalities.
This advantage addresses critical problems in Raman-including
resolution and intensity comparisons in Raman images-while
permitting on-line functional characterization such as thermal
conductivity, elasticity and adhesion, electrical and other
properties. (For more about the Nanonics platform, see section
on "Characteristics of integrated AFM and micro-Raman".)
Facilitating this integration is a new generation of AFM probes
with unique characteristics, such as hollow glass probes with
cantilevered nano-pipettes for material deposition; probes
with glass surrounding a single nano-wire for ultrasensitive
electrical measurements; or dualwire glass probes for thermal
conductivity and thermocouple measurements. Glass probes are
ideal for Raman integration because of their transparency
to laser light and lack of Raman background. They also expand
outward, allowing unprecedented correlation of Raman and AFM
and permitting multiple probes to be brought together easily,
which is very difficult with a standard AFM.
Negative stiffness vibration isolation
technology offers transmissibility advantages at multiple
frequencies compared with high performance air tables.
Source: Minus K Technology Inc.
Negative-stiffness vibration isolation
Underlying this pioneering integration is negative-stiffness
vibration isolation, developed by Minus K Technology Inc.
Improved transmissibility of a negative-stiffness isolator-that
is the vibrations that transmit through the isolator relative
to the input floor vibrations-is a particular benefit to SPM-Raman
and other NSOM systems, offering substantial improvement over
air systems and active isolation systems.
Negative-stiffness isolators employ a completely mechanical
concept in low-frequency vibration isolation. Vertical-motion
isolation is provided by a stiff spring that supports a weight
load, combined with a negative-stiffness mechanism. The net
vertical stiffness is made very low without affecting the
static load-supporting capability of the spring.
Beam-columns connected in series with the vertical-motion
isolator provide horizontal motion isolation. The horizontal
stiffness of the beam-columns is reduced by the "beamcolumn"
effect (a beam-column behaves as a spring combined with a
negative-stiffness mechanism). The result is a compact passive
isolator capable of very low vertical and horizontal natural
frequencies and very high internal structural frequencies.
The isolators (adjusted to 1/2 Hz) achieve 93% isolation efficiency
at 2 Hz; 99% at 5 Hz; and 99.7% at 10 Hz.
Aaron Lewis, president of Nanonics Imaging (the pioneer in
integrated AFM/micro-Raman), concludes: "Before negative-stiffness
vibration isolation was employed, AFM used in conjunction
with micro-Raman systems could not maintain adequate imaging
integrity while measuring at the nano-scale level. Vibration
isolation is absolutely necessary for the system's successful
performance, and negative-stiffness isolation has enabled
AFM and micro-Raman to function as a truly integrated platform."
Characteristics of integrated AFM and micro-Raman
As discussed earlier in the article, the integration of AFM
and micro-Raman offers key advantages, bringing micro-Raman
into the nanoscale world. Integrated platforms provide for
new avenues of improved resolution, including AFM functioning
without optical obstruction; parallel recording with Raman
in a wide variety of scanned probe imaging modalities (enabling
direct and simultaneous image comparison and analysis); and
high-resolution Raman mapping.
"Until recently, Raman scattering has remained separate
and removed from the proliferation of insights that the scanned
probe microscopies can give," says Aaron Lewis, president
of Nanonics Imaging, an Israel-based firm that was the first
to see the potential of such integration (www.nanonics.co.il).
"Without this integration of the systems, investigating
a sample with scanned probe microscopy required removing the
sample from the micro-Raman spectrometer. This meant that
the exact region that was being interrogated by Raman could
not be effectively correlated with the chosen SPM imaging
"Another aspect of optical integration is that SPMs can
measure forces, but they cannot measure distribution of light
in micro-lasers, silicon-based wave guides, fluorescently
stained biological materials, etc. For example, there are
many important advances occurring in the application of photonics
to silicon structures and plasmonic metals. In the past, these
photonic structures were in the micrometer range; now they
The Nanonics platform can be used for structural and photonic
characterization, as well as the structural and chemical characterization
that is available with AFM and Raman integration.
For these applications, Nanonics Imaging is the innovator
of AFM and NSOM systems, including dual tip/sample scanning
AFM systems; the industry's first NSOM-AFM cryogenic systems;
integrated Raman-AFM systems; and multi-probe AFM and SEM-AFM
systems. The company also holds patents to the largest range
of unique nano-probes. These probes form a NanoToolKit for
unique characterization platforms with a variety of tasks,
such as for nano-photonics, plasmonics, nano-chemical imaging
and even nano-chemical deposition based on its singular NanoFountainPen
technology. The company is focused on full integration of
AFM technology with optics, chemical imaging and other analytical
The Nanonics MultiView AFM-NSOM microscope, with its free
optical axis on a standard micro-Raman, now makes it possible
to truly integrate the separate worlds of Raman and AFM/NSOM
nanocharacterization, which has led to a new era in high-resolution
Jim McMahon writes on instrumentation technology. This
article was provided by Minus K Technology Inc., a company
founded in 1993 to develop, manufacture and market vibration
isolation products based on patented negative stiffness-mechanism
technology (www.minusk.com). Applications include nanotechnology,
biological sciences, semiconductors, materials research, zero-g
simulation of spacecraft, and high-end audio.
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